\documentclass{standalone}
\usepackage{tikz}
\usepackage{ctex,siunitx}
\setCJKmainfont{Noto Serif CJK SC}
\usepackage{tkz-euclide}
\usepackage{amsmath}
\usepackage{wasysym}
\usepackage{circuitikz}
\usetikzlibrary{patterns, calc}
\usetikzlibrary {decorations.pathmorphing, decorations.pathreplacing, decorations.shapes,}
\ctikzset{bipoles/length=1cm}
\begin{document}
\small
\begin{circuitikz}[>=latex, scale=0.8,european]
  % \useasboundingbox(0.9,0)rectangle(5.1,5);
  \draw (0,0) -- (5,0) to [R=$R_2$] (5,4) to  [R=$R_1$]  (0,4);
  \draw (5, 1)--(6,1)--(6,3)--(5,3);
  \draw[<->](0,.1)--(0,4-.1)node [midway,right]{$U$} ;
  \node at (6.3, 2){$L$};
  \draw [fill=white](0,4) circle (2pt);
  \draw [fill=white](0,0) circle (2pt);
  \draw [fill=black](5,1) circle (2pt);
  \draw [fill=black](5,3) circle (2pt);
\end{circuitikz}
\end{document}